Flanged terminal pins for dc/dc converters

ABSTRACT

A dc/dc converter is mounted to a printed circuit board with rigid terminal pins which extend into a converter substrate to provide electrical connection to circuitry on the substrate. A terminal pin includes a flange which abuts the printed circuit board and spaces the converter substrate from the printed circuit board. Connection to the printed circuit board is made by solder provided between the flange and the circuit board.

RELATED APPLICATIONS

[0001] This application is a Continuation-in-Part of application Ser.No. 09/740,707, filed on Dec. 19, 2000, which claims the benefit of U.S.Provisional Application No. 60/172,882, filed on Dec. 20, 1999. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] Designers are increasingly using distributed power supplyarchitectures for large electronic equipment. With this type-ofarchitecture, electrical power is bussed throughout the equipment at arelatively high dc voltage, such as 48 volts. dc/dc converters mountednear the load (and often on the same printed circuit board as the load)then step this high voltage down to the low voltage required by the load(e.g. 3.3V), typically through an isolating transformer.

[0003] These point-of-load dc/dc converters typically have a low height(e.g. 0.5″) so that the designer can place adjacent load boards closetogether in a card rack. The plan-view size of the converter must alsobe as small as possible to leave more room on the load board for theload circuitry. Several standard sizes of converters exist, such as the“Full Brick” (2.4″×4.6″), the “Half-Brick” (2.4″×2.3″), and the“Quarter-Brick” (2.4″×1.45″). Other standard and non-standard sizesexist, as well. In general, the larger a dc/dc converter, the more powerit can handle.

[0004] Typically, dc/dc converters have a flat bottom surface formed byeither a housing or potting material. Terminal pins extend from thissurface so that the dc/dc converters can be “through-hole mounted” on aprinted circuit board (the “PCB”). When the converter's “through-holepins” are inserted into the PCB's holes, the bottom surface of theconverter makes contact with the PCB to ensure its proper positioning inthe z-axis direction.

[0005] Recently, “open frame” converters have been developed without ahousing or potting. To achieve proper z-axis positioning, theseconverters use plastic or metal “standoffs” that keep the PCB and theconverter's substrate separated by a specified distance. Because thesestandoffs either abut or are attached to the converter's substrate, theytake up space on the substrate that could otherwise be used forelectronic components. They also partially or totally block the coolingair from flowing under the open frame converter. Finally, the standoffrepresents an additional cost for the part and for its attachment to theconverter.

[0006] Most electronic equipment manufactured today uses Surface MountTechnology (SMT) to attach their components to both the top and bottomsurfaces of a PCB. In this process, solder paste is first screen-printedonto the PCB in the locations of the component pads. The components arethen placed onto the solder paste. Finally, the PCB is passed through areflow oven in which the solder paste melts and then solidifies duringthe cool-down stage.

[0007] In comparison, dc/dc converters, with their through-hole pins,are attached to the PCB by either manual soldering or by an automatedproduction process called “wave soldering”. With this latter process,the PCB is first preheated and then passed over a molten pool of solder.The solder comes in contact with the bottom of the PCB, and it wicksinto the through-holes and solidifies after the PCB leaves the pool ofsolder.

[0008] A typical manufacturing process that requires both SMT and wavesoldering would first attach the SMT parts on the PCB, then insert thethrough-hole components, and finally pass the PCB through the wavesoldering machine. This process requires that the SMT components mountedon the bottom side of the PCB pass through a molten pool of solder.

[0009] As the distance between the leads on SMT packages gets smaller,it becomes more difficult to pass these packages through a wave solderprocess and not have solder bridges form between adjacent leads.Furthermore, the heating associated with the wave soldering processcompromises the quality of the SMT components and their attachments tothe PCB. Manufacturers of electronic equipment are therefore interestedin avoiding the use of wave soldering altogether. Often, the dc/dcconverter is the only component on their boards that requires wavesoldering.

[0010] In response to this desire, several power supply manufacturershave created dc/dc converters designed to be surface mounted to a PCB.Instead of a few, large diameter through-hole pins, some of theseconverters have many smaller leads designed for surface mounting. Ingeneral, these surface mount pins make a dc/dc converter's overallfootprint larger than it might otherwise be since the pins typicallyextend beyond the converter's original footprint. Alternatively, atleast one manufacturer has introduced a product that uses a surfacemountable ball-grid. In this product, each through-hole pin of astandard converter is replaced with a conductive ball of sufficientdiameter to permit the converter to be attached to the PCB with SMTtechniques.

[0011] One important problem with all of these approaches for making asurface mountable dc/dc converter is the relative weakness of a surfacemount joint compared to a through-hole pin. This problem is particularlyimportant since dc/dc converters have a higher mass than mostcomponents, and the mounting joints are therefore more susceptible toshock and vibration stresses.

[0012] Another problem with a surface mountable dc/dc converter is thatthe converter's pins make electrical contact with only the outerconductive layer in the PCB. Normally, the PCB's power and ground planesuse inner conductive layers. With a surface mount connection, additionalvias (that take up space and add resistance) are therefore required toconnect the outer conductive layer to the inner ones.

[0013] In comparison, a through-hole mounting is much strongermechanically. It also provides direct electrical attachment of the pinto the inner conductor layers of the PCB.

[0014] What is needed is a way to solder a through-hole mounted pin witha reflow solder process, instead of using manual or wave soldering.

SUMMARY OF THE INVENTION

[0015] To address the problems mentioned above, a new through-holeterminal pin is used for mounting dc/dc converters or other circuitmodules. In one embodiment, this pin is similar to a standardthrough-hole pin, but it has a circular flange near its bottom end. Thediameter of the flange is greater than the diameter of the PCB holethrough which the lower portion of the pin is inserted. The bottom ofthe flange therefore rests against the PCB's surface. It is located aspecified distance from the dc/dc converter's substrate so that itprovides the function of a stand-off, but without taking up space on thesubstrate or requiring a separate part. In addition, its interferencewith the cooling airflow underneath the dc/dc converter is minimal.

[0016] In another embodiment, the through-hole pin has a flange near orat the top end of the pin where it makes contact with the dc/dcconverter's substrate. The top of this flange rests against the bottomof the substrate. This arrangement improves the mechanical connection ofthe pin to the dc/dc converter's substrate, and it provides one way toensure the proper z-axis placement of the pin relative to the substrate.

[0017] In a third embodiment, the through-hole pin has one continuous,larger diameter portion that performs the function of separate flangeson either end.

[0018] In a fourth embodiment, the end of the pin has a cross-sectionalshape that is pointed along its periphery. This pointed shapefacilitates press fitting, or swaging, the pin into a hole of either thesubstrate, the PCB, or both. The press fit holds the pin in place forlater soldering in a hand, wave, or reflow process, and it improves themechanical strength between the pin and the substrate or PCB.

[0019] In addition, a process has been invented to permit this newthrough-hole pin to be soldered to the PCB with a reflow process,instead of using manual or wave soldering. In one embodiment, thisprocess works as follows.

[0020] First, the pad around the PCB's through-hole is designed to becommensurate in size and shape with the flange of the converter pin.

[0021] Second, solder paste is screen-printed onto the PCB in thelocations of the pads for both the SMT components and the dc/dcconverter pins.

[0022] Third, both the SMT components and the dc/dc converter are placedon the PCB. The dc/dc converter, since it is relatively large and heavy,might be placed manually or by a special machine, although it could beplaced by the same machine as the other SMT components. At this point,the bottoms of the flanges sit on top of solder paste, while the lowerparts of the through-hole pins are inserted into their PCB holes.

[0023] Finally, the PCB is passed through a reflow oven in which thesolder paste first melts and then solidifies. During this step, thesolder paste between each pin flange and the PCB wicks down into thecorresponding PCB hole. The final solder joint between the pin and thePCB will therefore exist both underneath the flange and inside the PCBhole. With a properly designed pad and screening process, there willalso be a fillet of solder around the outer edge of the flange toprovide additional mechanical stress relief. The result is a very strongmechanical connection between the pin and the PCB, as well as a lowresistance electrical connection between the pin and both the inner andouter conductive layers of the PCB.

[0024] The flange facilitates this special soldering process. Itprovides a region in which the solder paste directly contacts both thepin and the PCB. As the solder melts, it readily wicks along the surfaceof the flange and down the pin such that it fills the gap between thepin and the PCB hole's via metalization.

[0025] In another embodiment of the reflow soldering process, the bottomend of the through-hole pin is given a cross-sectional shape that ispointed. When the pin is press fit into the PCB, the points of the pinhold the pin, and therefore the converter, in place. Solder is thenapplied to the bottom side of the PCB in the region of the hole and itspad. The PCB is then passed through a reflow oven in which the solderpaste melts, flows into the gaps between the pin and the hole, and thensolidifies.

[0026] In this alternative reflow soldering process the end of theinserted pin should not extend beyond the bottom of the PCB. Otherwise,it might interfere with the solder application step. In fact, it isuseful for the end of the inserted pin not to reach the bottom side ofthe PCB (i.e., for the end to be inside the PCB). Such an alignmentgives a small “well” in the hole area, which increases the amount ofsolder that can be applied in this area. A flange near the bottom end ofthe pin facilitates the correct insertion depth of the pin into the PCBhole, although there are other well-known means for controlling thisdepth.

[0027] This alternative reflow soldering process can also be used toattach the pin to the dc/dc converter's substrate during theconstruction of the converter.

[0028] Thus, in accordance with one aspect of the invention, a dc/dcconverter comprises a converter substrate having circuitry thereon. Atleast one rigid terminal pin directly attaches to the convertersubstrate and is electrically connected to the circuitry. The terminalpin includes a flange having a shoulder to abut a printed circuit boardinto which the pin is inserted and to which electrical connection ismade. The shoulder may abut the printed circuit board by making directcontact thereto, or through one or more layers of material, such assolder. The shoulder is spaced from the converter substrate toaccommodate spacing of the converter substrate from the printed circuitboard. Plural pins may together provide the spacing between theconverter substrate and the printed circuit board or one or more pinsmay operate with more conventional standoff mechanisms.

[0029] To allow for a subsequent soldering process to, for example,solder the terminal pin to the printed circuit board, the components,materials and solder connections of the converter may be such that theyare not adversely affected by a 210° soldering process. In particular,the solder used on the converter substrate has a melting temperaturegreater than 210° C.

[0030] The terminal pin may have a second shoulder which abuts theconverter substrate. For example, the second shoulder may be on a secondflange with the pin extending from the second flange into the convertersubstrate. The second flange may be spaced from the first flange.Alternatively, a single flange may extend along a length of a terminalpin to abut both the printed circuit board and the converter substrate.In one embodiment, the flange has a uniform diameter.

[0031] The terminal pin may be swage fit into the converter substrate.To that end, the pin may have a pointed cross-section shape. The portionof the terminal pin extending into the converter substrate may also besoldered to the converter substrate such as by a reflow solderingprocess.

[0032] The invention is particularly suited to a converter substratehaving circuitry thereon in an open frame construction without abaseplate where the converter substrate is positioned parallel to theprinted circuit board.

[0033] In accordance with another aspect of the invention, a dc/dc powerconverter is mounted to a printed circuit board by soldering theconverter to the printed circuit board with the terminal pin extendingthrough a circuit board hole and the shoulder of the terminal pinabutting the circuit board to accommodate spacing of the dc/dc converterfrom the circuit board. Preferably, the solder is applied to the circuitboard or shoulder, and the shoulder is thereafter positioned to abut theprinted circuit board through the solder. The solder may be applied as asolder paste about the circuit board hole, and the hole may be leftsubstantially free of solder paste when the paste is applied. Theassembly may thereafter be subjected to a solder reflow process.

[0034] The solder may flow to form a fillet. For example, the solder mayflow radially to form a fillet about the flange. The solder may alsoflow through the hole in the printed circuit board to form a filletabout a portion of the terminal pin exposed beyond the circuit boardhole.

[0035] Solder may be applied to the holes from an opposite board side ofthe printed circuit board after insertion of the terminal pins into theholes. Specifically, the solder may be applied from a molten pool ofsolder positioned below the printed circuit board.

[0036] In accordance with another aspect of the invention, solder ispreapplied on the shoulder of the flange. For example, the solder on theflange may be in a paste, may be a preform, or may be coated on theshoulder of the flange.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0038]FIG. 1 illustrates a typical dc/dc converter with a housing orpotting and through-hole pins.

[0039]FIG. 2 illustrates an open-frame dc/dc converter (having nohousing or potting) that displays one example of a standoff structure.

[0040]FIG. 3 illustrates an open-frame dc/dc converter without a metalbaseplate that displays another example of a standoff structure.

[0041]FIG. 4 illustrates a through-hole pin with a flange.

[0042]FIGS. 5a-c each illustrate a through-hole pin with a flange and anend with a cross-sectional shape that is pointed.

[0043]FIG. 6 illustrates a through-hole pin with two flanges.

[0044]FIG. 7 illustrates a through-hole pin with a single flange thatsits against both the PCB and the substrate.

[0045]FIG. 8 illustrates a through-hole pin with two flanges where thetop flange is flush with the top end of the pin.

[0046]FIGS. 9a-e illustrate using a reflow process to solder a flangedthrough-hole pin to a PCB or a substrate.

[0047]FIGS. 10a-c illustrate using a reflow process to solder apress-fit through-hole pin (having a cross-sectional pointed shape atits end) to a PCB or a substrate.

[0048]FIG. 11 is a cross-sectional view of a converter module mounted toa printed circuit board in accordance with the invention.

[0049] FIGS. 12A-12B illustrate a pin having a chamfered flange.

[0050] FIGS. 13A-13C illustrates the flanged pin in a wave solderingprocess.

[0051] FIGS. 14A-14B illustrate an alternative flange in which the sidesof the flange are completely shaved.

[0052] FIGS. 15A-15B illustrate a chamfered flange pin in a vibratoryfeeder.

[0053] FIGS. 16A-16C show a flanged pin in a reflow solder process.

[0054]FIG. 17 illustrates a solder joint resulting from a reflowprocess.

DETAILED DESCRIPTION OF THE INVENTION

[0055] A description of preferred embodiments of the invention follows.

[0056] Throughout this discussion and in the figures we will assume thecross-section of the pin and its flange is circular. One skilled in theart would know how to incorporate the concepts presented here for othercross-sectional shapes, such as triangular or rectangular.

[0057]FIG. 1 shows a typical dc/dc converter 100 with a metal baseplate101 (to which a heatsink might be attached), a housing or potting 102(inside which is the converter's circuitry), and its through-hole pins103. The pins have various diameters (e.g. 40, 60, and 80 mils) tohandle their rated current, and various lengths below the housing (e.g.110, 145, and 180 mils) to pass all the way through the PCB holes.

[0058]FIG. 2 shows an open-frame dc/dc converter 200 with a metalbaseplate 201 and through-hole pins 203. Since there is no housing orpotting in this converter, the converter's circuitry 202 is visible. Insome open-frame converters, the circuitry is mounted on a singlesubstrate, and in other converters, two substrates are used. In eithercase, one substrate (the “baseplate substrate”) is either part of, orattached to, the metal baseplate so that the heat dissipated by thepower components on this substrate can readily flow to the baseplate.

[0059]FIG. 2 also shows a typical standoff structure 204 that is used onthe converter with no housing or potting. Standoff 204 is typically madeof plastic and is designed to abut the baseplate substrate. As can beseen from this figure, the standoff requires clear space (free ofcomponents) on the baseplate substrate. It also reduces the availablespace for other substrates and their components in the dc/dc converter.

[0060]FIG. 3 shows another open-frame dc/dc converter 300 that does nothave a metal baseplate. Standoffs 301 are mounted on this converter'ssingle substrate 302, along with the converter's circuitry. The spacethese standoffs take is not available to circuit components.Through-hole pins 303 are attached to substrate 302 using either athrough-hole or a surface mount technique.

[0061]FIG. 4 shows a new through-hole pin 400 for dc/dc converters. Thispin has a shank 401 (in this case circular with a typical diameter of 80mils) and it has a flange 402 located along the length of the shank. Asshown in this embodiment, the flange is circular with a diameter and athickness that may, for example, be 120 mils and 40 mils, respectively.The flange diameter is larger than the diameter of the hole in the PCBso that when the bottom portion 406 of the pin (between 403 and 404) isinserted into the hole, the bottom side of the flange makes contact withthe top of the PCB.

[0062] The bottom side 403 of the flange is located a specific distancefrom the bottom end 404 of the pin. The length of portion 406 is chosensuch that the bottom end of the pin will pass all of the way through thePCB. Typical lengths for 406 are 110 mils, 145 mils, and 180 mils, eachchosen to accommodate a different thickness PCB.

[0063] Through-hole pin 400 has its top end designed to be through-holemounted to the dc/dc converter's substrate, as well. The length of thetop portion 407 of the pin (between 403 and 405) and the depth to whichportion 407 is inserted into the hole of the converter substrate arechosen such that the bottom side 403 of the flange is located a specificdistance from the substrate. By doing this, the bottom side of theflange will hold the dc/dc converter substrate this specified distanceabove the PCB, thereby performing the function of a standoff.

[0064] The ends 404 and 405 of the pin can have various shapes, such asconical or spherical, to facilitate the manufacture of the pin and theinsertion of the pin into its mounting holes.

[0065] The top portion 407 of the pin may have design features thatfacilitate its mounting to the substrate. For instance, the pin might bepress fitted (or swaged) into the substrate's hole to hold it in placeuntil it is soldered and to provide a greater mechanical strength evenafter it is soldered. If the cross-sectional design of 407 is circular,however, it would make contact with the side of the substrate holearound the entire perimeter. This tight fitting would not allow solderto wick down between the pin and the hole to provide a reliableelectrical connection between the pin and the inner conductor layers ofthe substrate.

[0066]FIGS. 5a and 5 b show an alternative cross-sectional design forthe upper portion 501 of portion 407 of the pin. The points 502 of thehexagonal design for 501 allow the pin to be press fit into thesubstrate hole while still leaving spaces 503 for the solder to wickdown into the hole. Other “pointed cross-sectional shapes,” shapes whichleave open space about the periphery between the pins and the side wallof the hole, such as other polygons or star-shapes, could accomplish thesame function.

[0067] Similarly, part or the entire bottom portion 406 of the pin couldbe given a pointed shape so the dc/dc converter pin could be press fitinto the PCB and then soldered. FIG. 5c shows an example 504 of such apin design.

[0068] One way to manufacture a pin with a cross-sectional pointed shapeat its top and/or bottom end is to start with a shank of the desiredcross-sectional shape. Another way is coin, stamp, impact-extrude, orturn on a screw machine to give an end of the pin its desired pointedshape.

[0069] To facilitate the mounting of the pin to the dc/dc converter'ssubstrate, the pin could have another flange near the top end of thepin, as FIG. 6 shows. The pin could be inserted or press fit into thesubstrate hole until the topside 603 of the top flange 602 makes contactwith the substrate. Flange 602 would thereby ensure that the pin isinserted (or press fit) the correct distance into the substrate hole. Italso provides additional mechanical strength to the connection betweenthe pin and the substrate, as well as additional electrical connectionbetween the two.

[0070] Another variation to the pin is shown in FIG. 7. In this figure,the functions of both the bottom flange 402 and the top flange 602 areaccomplished with a single flange 702. The standoff distance requiredbetween the substrate and the PCB determines the length of flange 702.This single-flange pin 700 can be easier to manufacture, have greatermechanical strength, and lower electrical and thermal resistance than atwo-flange pin design.

[0071] Another variation to the pin is shown in FIG. 8. In thisembodiment, the connection between the pin and the dc/dc convertersubstrate uses a surface mount, rather than a through-hole, technique.Flange 602 is now flush with the top end of the pin. As such, its topsurface, 801, provides a flat surface that can be soldered to thesubstrate with an SMT process. In another embodiment, by combining theconcepts depicted in FIGS. 7 and 8, the pin would use a single flangewith the top of the flange now flush with the top end of the pin.

[0072] The new through-hole pin described above can be wave- orhand-soldered to the PCB. It can also be reflow-soldered to the PCB witha process similar to that used for SMT components. As such, the new pincombines the mechanical and electrical advantages of a through-hole pinwith the convenience and compatibility of an SMT pin.

[0073] The method by which the new pin can be reflow-soldered to the PCBis as follows.

[0074] First, as shown in FIG. 9a, a pad 901 of exposed conductor aroundthe hole 902 in the PCB 909 is made slightly larger in diameter than thediameter of the flange 903. Second, as shown in FIG. 9b, solder paste904 is applied to pad 901. Third, the dc/dc converter is placed on thePCB such that pin 900 is inserted into hole 902 until the bottom offlange 903 rests on the solder paste 904, as shown in FIG. 9c. The PCBand dc/dc converter are then passed through a reflow oven, which raisesthe temperature of everything until the solder past melts. Once melted,the solder wicks both down into the hole 902 and up the side of theflange 903. Finally, the solder is allowed to cool and solidify. Theresult, shown in FIG. 9d, is a solder joint (or connection) 905 betweenthe pin and the PCB that exists within the hole, underneath the flangeand along the side of the flange. The “fillet region” 906 of the solderalong the side of the flange provides additional mechanical strength tothe solder connection and provides visual assurance that the solder hasfilled the region between the flange and pad. For best performance, thefillet should have a concave shape, as shown in the figure. Similarly,there should be a fillet 907 where the pin protrudes through the PCB.

[0075] A typical way to apply the solder paste 904 to the pad 901 is toscreen-print it onto the pad at the same time that solder paste isscreen-printed onto the pads for the PCB's other SMT components.However, pad 901 has a hole in the center of it and it is preferable tonot screen-print solder paste over this hole. FIG. 9e shows one way toconfigure the opening 907 in the screen-printing stencil 910 to achievethis result.

[0076] It is important to apply sufficient solder paste to the pad 901so that the solder connection will be electrically and mechanicallysound. It is also important to avoid too much paste, although thiscondition is generally less of a problem.

[0077] Depending on the size of the dc/dc converter's pin and its PCBhole, the amount of paste desired may be more than the amount applied bya screen-printing process that works for the other, SMT components onthe PCB. One way to get additional solder paste on pad 901 is to“overprint” the solder paste. With this approach, the opening 907 in thescreen-printing stencil 910 is larger in diameter than the pad 901. Thesolder paste printed outside the pad area initially sits on top of thesolder mask 908. During the reflow process, as this solder paste meltsit is drawn off the solder mask and into the desired solder joint regionby the action of surface tension.

[0078] Another way to apply the correct amount of solder paste on pad901 is to dispense it through a needle, rather than screen-print it.This dispensing process could be either manual or automatic.

[0079] A third way to apply the correct amount of solder on pad 901 isto use a “solder preform”, which is a thin sheet of solidified solderthat has the desired shape and thickness and total volume of solder.This preform can be applied to pad 901 with either a manual or automaticprocess.

[0080] Another way to apply the solder is to preapply it directly to theshoulder of the flange before the flange is positioned against the pad.For example, the solder could be coated on the underside of the flange,could be applied as a paste, or could be applied as a solder preform.The solder could be preapplied by the final installer or could bepreapplied by the pin or converter manufacturer.

[0081] With both the dispensing and the pre-form approaches for applyingsolder, it is again possible for the solder to extend initially beyondthe pad 901 and to sit on top of the solder mask 908. As with theoverprinting approach, the solder will be drawn off the solder mask andinto the solder joint region by surface tension during the reflowprocess.

[0082] Some experimentation will be required to determine how muchsolder paste should be applied in a given situation. The amount willdepend on issues such as which solder application method is chosen, thesize of the pin, its flange, and the hole, the thickness of the PCB, thenumber and thickness of the conductors in the PCB, the details of thereflow process, etc. An SMT process engineer of ordinary skill in theart would generally be able to determine a good starting point for thisexperimentation. Then, by mechanically inspecting the resultant solderconnection between the pin and the PCB, the engineer could easilydetermine whether the amount of solder used was too little or too much.In this manner, a final solution could be found after just a fewiterations.

[0083] As an example of how much solder might be used, consider thefollowing:

[0084] 1) shank diameter=80 mils

[0085] 2) flange diameter=120 mils

[0086] 3) flange thickness=40 mils

[0087] 4) hole diameter=90 mils

[0088] 5) pad diameter=160 mils

[0089] 6) PCB thickness=90 mils

[0090] 7) 6 layers of 4 oz. and 2 layers of 2 oz. copper within the PCB

[0091] 8) reflow process: 5 min ramp-up time, 210° C. peak temp for 1min, 2 min ramp-down time

[0092] For this situation it has been determined that a solder volume of106 cubic mils gave a good solder connection.

[0093] Because the dc/dc converter and its pins usually have a higherthermal mass than other components on the PCB, the ramp-up time and theramp-down times in the reflow oven might need to be increased over thevalues used if the dc/dc converter were not present.

[0094] Since the dc/dc converter will be passed through a reflow oven,it is important to ensure that the converter's components, materials,and solder connections are not adversely affected during this process.For instance, the converter might be fabricated with higher temperaturesolder than the one used to attach the converter to the PCB. A PCBsubstrate within the converter might have a higher temperature rating(e.g. 150° C. or 185° C.) instead of the normal 130° C. rating.

[0095] A typical solder which would be used to join the terminal pin tothe PCB has a melting temperature of 183° C. Thus, the conditions of thereflow oven are such that a peak temperature of the solder reaches about210° C. as noted above at point 8. In order to assure the integrity ofthe dc/dc converter it is preferred that the solder used in theconverter have a melting temperature greater than 210° C. Preferably, asolder having a melting temperature greater than 230° C. is used in theconverter assembly.

[0096] A second method by which the new pin can be reflow-soldered tothe PCB is as follows.

[0097] First, as shown in FIG. 10a, the bottom portion 1001 of the pin1000 has a pointed shape to its cross-section so that it can be pressfit into the PCB 1002.

[0098] Second, when the pin is inserted into the PCB hole, the depth ofthe insertion is controlled to keep the bottom end 1003 of the pin fromextending beyond the bottom surface 1004 of the PCB. Preferably, thebottom end of the pin should not reach the bottom PCB surface, butinstead remain slightly (e.g. 15 mils) inside the PCB, as shown in FIG.10b. FIG. 10b shows a flange 1005 near the bottom end of the pin thatfacilitates the correct insertion depth, although other means well knownto those skilled in the art could be used instead. For instance, amachine could be used to insert the pins, and the range of the machine'smotion could then be controlled to achieve the correct insertion depth.

[0099] Third, with the dc/dc converter held in place by the press-fitpin, solder paste can be screen printed onto the bottom side of the PCB,as shown in FIG. 10c. Since the end 1003 of the pin does not extendbeyond the bottom surface 1004 of the PCB, the pin does not interferewith this screen printing. In addition, by leaving the end of theinserted pin slightly (e.g. 15 mils) inside the PCB, a slight “well”1006 is formed in the area of the hole. During the screen printingprocess, this well is filled with solder paste. The dimensions of thewell can therefore be adjusted to achieve the desired amount of solderpaste.

[0100] At this time, other SMT components can be placed on the bottomside of the PCB.

[0101] The PCB is then passed through a reflow oven where the soldermelts, flows down into the gaps between the pin and the hole, and thensolidifies.

[0102] This same method can be used to solder the pin to the dc/dcconverter's substrate during the construction of the converter. The pinin this figure does not have a flange near the end of the pin that isinserted into the substrate, although it might.

[0103]FIG. 11 shows how the final assembly might look in cross-section.The open-frame dc/dc converter has a substrate 1101 on which circuitry1102 is attached. Only a few circuit components are shown in this figurefor simplicity. In general, there would be many components mounted onboth sides of the converter substrate 1101.

[0104] Several terminal pins 1103 with flanges 1105 are swage fit intoholes 1106 of the converter substrate. These pins are then soldered tothe holes in the spaces 1107 between the pins and the side walls of theholes. Conductive traces 1108 on the converter substrate 1101electrically connect the terminal pins to circuitry 1102.

[0105] The other end of pins 1103 are inserted into the printed circuitboard 1 104. The shoulder of the flange 1105 of these pins abuts theprinted circuit board.

[0106] Solder 1109 connects the pin 1103 and its flange 1105 toconductive pads on the printed circuit board and the sidewalls of theholes. Fillets 1110 of solder are formed around the flanges and aroundthe end of the pin that extends through the printed circuit board.

[0107] The terminal pins 1103 are connected electrically to other partsof the printed circuit board through conductive traces 1111.

[0108] Modifications to the flanges in the pins are shown in FIGS.12-17.

[0109] As shown in FIGS. 12A and B, chamfers 1202, 1203 were cut oneither side of the pin flange 1201. The root of the chamfer is tangentto (1205, 1206) the shank of the pin and extends upward at an angle1204, in this case 45 Deg., though other angles could be used. Theselection of the chamfer design was made so that the amount of remainingstandoff area 1207 could be maximized for proper support of the unit.The primary purpose of these chamfers is to reduce the opportunity forvoid formation during the process of soldering the pin into the enduser's printed circuit board (EUPCB).

[0110] The typical process for soldering the pin into the EUPCB is wavesoldering. The converter is installed into the EUPCB manually or bymachine. It is then placed on the conveyor of the wave solder machine.The EUPCB moves through the several zones of the wave solder machine.Flux is applied to the bottom of the EUPCB using a spray or foam. Theflux may be a No Clean or Water Soluble formulation, and serves to coatthe solderable surfaces on the pin and EUPCB. Next the EUPCB travelsthough the preheat zone, where the flux is activated and breaks downsurface oxides on the solderable surfaces. The EUPCB then travelsthrough the waves of molten solder, typically two, one with laminar flowand one with turbulent flow. During this process some of the fluxcomponents evolve as gasses due to the magnitude and rapid change intemperature as the waves are crossed. The EUPCB then exits the wavesoldering machine and begins to cool.

[0111]FIG. 13A shows a cross-section of a pin 1301 seated in an EUPCB1303 during wave soldering. During the wave soldering process asituation can arise in which the rapidly evolving gasses 1305 mightbecome trapped beneath the flange 1302 on the pin 1301, given therectangular cross-section without chamfers, in the following manner. Thetrapped gasses 1305, having no means of escape because the flange 1302forms a seal 1306 at the interface between the flange 1302 and thecopper barrel of the through hole 1304 on the EUPCB 1303, create abarrier for the advancement of the liquid solder 1307 up the shank ofthe pin and prevent during the wave soldering process and prevents thesolder 1307 from flowing between the flange 1302 and the barrel of thethrough hole 1304 to form a fillet 1308 around the flange 1302.

[0112]FIG. 13B shows how the new pin enables the escape of the fluxgasses. The chamfers 1309 in the flange 1302 on two opposing sides allowany trapped gasses 1305 to be vented in advance of the molten solder1307 . As a result (FIG. 13C), a fillet 1308 can form on the top side ofthe plated through hole 1304 in the EUPCB 1303.

[0113] As shown in FIGS. 15A and B, the design of the chamfers does notaffect the ability for the pin 1500 to be fed automatically by vibratorybowl during insertion. The top surface 1208 of the flange 1201 is usedto stride the track 1501 of the vibratory feeder.

[0114] Other options for providing venting include shaving off the sidesof the flange completely as in FIGS. 14A and B. Such options, however,create problems for bowl feeding the pins 1400 because the ability toride the feed rails 1501 now becomes a function of orientation: When theaxis 1403 of this reduced flange 1401 is parallel to the rail 1501 slot,the pin can fall in and jam the bowl. The chamfers 1202, 1203 on thepins 1200 do not intersect the top surface 1208 of the pin 1200, so thatthe top side 1208 of the flange 1201 remains flat for riding the rails1501.

[0115] The chamfers 1202, 1203 in the flanges 1201 also facilitate theFlanged Pin in Paste (FPiP) reflow based process. Referring to FIG. 16A,which shows the cross-section of the pin 1601 in the EUPCB 1603 during areflow solder process. Solder paste 1601 is deposited over the barrel ofthe plated through hole 1604. Per FIG. 16B, the pin 1600 is insertedinto the hole 1604 through the paste 1601 until it rests on the barrel1604. The paste deposit 1601 separates into two regions, the annularring 1607 left on the hole 1604 and a slug 1608 that remains on the tipof the pin 1605.

[0116] The openings 1602 between the flanges 1609 and the barrel 1604 ofthe EUPCB 1603 allow movement of the deposited solder paste 1607 fromthe top side of the board through to the pin inside the barrel of thePTH during reflow. Paste from the top surface overprint and the slug ofpaste 1608 that is carried on the tip of the pin 1609 migrate togetherduring reflow, being drawn by surface tension and wetting forces alongthe shank of the pin without impediment. This opening 1602 also allowsfor a wetting of the pin 1601 and barrel plated through hole 1604 of theEUPCB 1603 that is continuous as this opening is not cut off when thepin is fully seated.

[0117] The resultant solder joint 1701 is shown in FIG. 17, with fillets1708 formed about the chamfered regions (1709), wetting the entire hole1704 and shank of the pin 1706.

[0118] The design of the chamfers also leaves a large flat surface 1207on the pin 1200 to bond with the annular ring of the plated through hole1304 on the EUPCB 1308 for a stronger surface joint on top of the PCB,and side fillets 1708 that form in the chamfered regions 1709, giveextra surface area for bonding. The amount of bonding surface parallelto the surface of the EUPCB is an important factor in the distributionof stresses at the interface of the pin and the solder. Stressdistributions are kept to a minimum and thereby decreasing the magnitudeof stains generated due to mechanical loads and thermal mismatch.

[0119] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method of mounting a dc/dc power converter on aprinted circuit board comprising: providing a dc/dc convertercomprising: a converter substrate having circuitry thereon; and at leastone rigid terminal pin attached to the converter substrate, the pinbeing electrically connected to the circuitry, the terminal pinincluding a flange having a shoulder spaced from the convertersubstrate; applying solder to at least one of the shoulder of the flangeand the circuit board; positioning the dc/dc converter to the printedcircuit board with the terminal pin extending through a circuit boardhole and the shoulder abutting the circuit board to accommodate spacingof the dc/dc converter from the circuit board with solder between theshoulder of the flange and the printed circuit board; and subjecting thedc/dc converter and printed circuit board to a solder reflow process tojoin the terminal pin and printed circuit board with the solder.
 2. Amethod as claimed in claim 1 wherein the dc/dc converter comprises asubstrate with circuitry thereon in an open frame construction.
 3. Amethod as claimed in claim 1 wherein the solder paste is applied as apaste about the circuit board hole.
 4. A method as claimed in claim 3wherein the hole is left substantially free of solder paste when thepaste is applied.
 5. A method as claimed in claim 1 wherein the solderflows within the hole of the printed circuit board.
 6. A method asclaimed in claim 1 wherein the solder flows to form a fillet.
 7. Amethod as claimed in claim 6 wherein the solder flows radially to form afillet about the flange.
 8. A method as claimed in claim 6 wherein thesolder flows through the hole in the printed circuit board to form afillet about a portion of the terminal pin exposed beyond the circuitboard hole.
 9. A method as claimed in claim 1 wherein the components,materials and solder connections of the converter are not adverselyaffected by a 210° C. soldering process.
 10. A method as claimed inclaim 1 wherein solder used on the converter substrate has a meltingtemperature greater than 210° C.
 11. A method as claimed in claim 1further comprising a second flange on the terminal pins that abuts theconverter substrate, and the pin extends from the second flange into theconverter substrate.
 12. A method as claimed in claim 1 wherein theflange extends along a length of the terminal pin to abut the convertersubstrate.
 13. A method as claimed in claim 1 wherein the terminal pincomprises a second shoulder which abuts the converter substrate.
 14. Amethod as claimed in claim 13 wherein the terminal pin extends into theconverter substrate.
 15. A method as claimed in claim 14 wherein theterminal pin is swage fit into the converter substrate.
 16. A method asclaimed in claim 15 wherein the portion of the terminal pin extendinginto the converter substrate has a pointed cross sectional shape.
 17. Amethod as claimed in claim 16 wherein the portion of the terminal pinextending into the converter substrate is soldered to the convertersubstrate.
 18. A method as claimed in claim 1 wherein the terminal pinextends into the converter substrate.
 19. A method as claimed in claim18 wherein the pin is swage fit into the converter substrate.
 20. Amethod as claimed in claim 19 wherein the portion of the pin extendinginto the converter substrate has a pointed cross section shape.
 21. Amethod as claimed in claim 20 wherein the portion of the terminal pinextending into the converter substrate is soldered to the convertersubstrate.
 22. A method as claimed in claim 1 further comprisingapplying solder on the shoulder of the flange prior to positioning theterminal pin in the circuit board hole.
 23. A method as claimed in claim22 wherein the solder is applied in a paste.
 24. A method as claimed inclaim 22 wherein the solder is applied as a preform.
 25. A method asclaimed in claim 22 wherein the solder is coated on the shoulder.
 26. Aterminal pin comprising: an elongated pin for insertion into a printedcircuit board; a flange having a shoulder to abut the printed circuitboard into which the pin is inserted; and preapplied solder on theshoulder of the flange.
 27. A terminal pin as claimed in claim 26wherein the solder is in a paste.
 28. A terminal pin as claimed in claim26 wherein the solder is a preform.
 29. A terminal pin as claimed inclaim 26 wherein the solder is coated on the shoulder.
 30. A circuitmodule comprising: a circuit substrate having circuitry thereon; and atleast one rigid terminal pin attached to the circuit substrate, the pinbeing electrically connected to the circuitry, the terminal pinincluding a flange having a shoulder to abut a printed circuit boardinto which the pin is inserted, the shoulder of the flange havingpreapplied solder thereon.
 31. A circuit assembly as claimed in claim 30wherein the circuitry implements a dc/dc converter.